With one exception (so far), I do my own art from scratch: covers, chapter headers, anything else that might be needed. I’m not a very good artist, but I’m good enough to get the point across, and good enough to generally create images that satisfy me. I do open the floor to feedback during the process, but ultimately, I tend to start with a particular vision and carry it through to the finished piece.
Part of the reason why I do my own art is because I do tend to have a certain artistic vision for my characters look like, and the way they behave and are written is tied into a lot of that vision. Of course, that’s my interpretation of the characters in question. They aren’t necessarily an iron decree that these characters must look like this in your head or else you’re doing it wrong. How I interpret my work informs how I create my work and iterate on it in sequels and the like, but it doesn’t dictate how you do so - as long as you aren’t entirely disregarding what’s written in the text.
That being said, my current project will probably be using outside commissions (because I do not have the chops to do this one justice), so it’ll be interesting to see how my vision intersects with the artist’s vision, and with the players’ vision.